1. Field of the Invention
The present invention relates to an ultrasonic cutting tool. More particularly, but not exclusively, it relates to a tool for cutting material, especially soft material such as flesh. It may be useful in a laparoscopic cutting system particularly for haemostatic cutting.
2. Discussion of the Background
It is known to cut tissue by means of ultrasonically vibrated knives or scalpels. When a scalpel cuts tissue its effectiveness is indicated by the cutting force. This derives from the pressure required to separate the structure and from the frictional drag as the blade is drawn between the cut sections. Vibrating the blade can reduce friction and may also reduce the bond strength of the tissue. Both objectives could be achieved by applying vibrations to the cutting blade in either a longitudinal or torsional mode.
However, only axial mode oscillatory systems have been used hitherto. These can produce appropriate conditions for soft tissue cutting, controlled coagulation and aspirated dissection. Longitudinal compression waves couple most efficiently to achieve effective transmission but we believe it is incorrect to assume that this mode is optimally deployed for a wide range of surgical applications. Caution must be exercised when applying ultrasound at intensities in the order of hundreds of watts per cm2.
It is now well known that longitudinal mode vibrations can safely be applied in specific orthopaedic procedures involving bone cement removal. In such cases it is appropriate to use longitudinal vibration modes since the acoustic properties of the prosthetic cement and bone tissue ensure minimal transmission from the operating site. However, this situation is unusual and generally it is necessary to take special precautions when using high intensity ultrasound to effect tissue removal in potentially vulnerable regions of the anatomy.
At frequencies in the low kHz range, pressure and displacement waves transmit deep into human tissue since absorption rates at these relatively low frequencies are quite low. However, due to the unpredictable transmission path, standing waves can occur leading to local increased absorption and heating. At energy levels applied in tissue cutting there is a significant risk of creating cell damage in areas remote from the operating site. Ultrasound energy is absorbed in three ways, by frictional heating due to relative cyclic motion at the instrument tissue interface by direct absorption in the molecular structure of the excited tissue and by cavitation. Frictional transmission produces local heating adjacent to the activated instrument surface and is inherently safe. However, cavitation and direct molecular absorption can lead to damaging effects which in extreme cases could be harmful.
Mechanical power transmission is defined by the product of oscillatory force and oscillatory velocity amplitudes, and in the case of a longitudinal mode system loaded in a direction normal to the axis of vibration, i.e. displacement parallel to a transmission interface, this is given by:
i PL=1/2Ffd"xgr"L/dtxe2x80x83xe2x80x83(1)
where PL is the r.m.s. power, Ff is the frictional force amplitude and "xgr"L is the particle displacement amplitude normal to the clamping force between blade and tissue. The frictional force acts in a direction parallel to the longitudinal vibration but is a function of the clamping force and a term defining the frictional drag between the blade and tissue at the transmission interface. This term may vary in time (correlated with the longitudinal vibration) between values of 0 and 1 representing zero or full coupling between blade and tissue.
For torsional mode, i.e. displacement normal to a transmission interface, power transmission is given by:
PT=1/2FTd"xgr"T/dtxe2x80x83xe2x80x83(2)
Where PT is the r.m.s. power, FT the oscillatory force amplitude and "xgr"T the torsional displacement amplitude.
In the case of a polished blade being pressed normally onto tissue it is reasonable to assume that the normal clamping force is small compared with the direct force amplitude FT. If the friction force Ff=0.1 FT for instance, then from equations (1) and (2):
PT=10PL,
assuming similar tissue loading conditions (blade tissue impedance), and d"xgr"L/dt=d"xgr"T/dt.
It is therefore evident that the energy transmission may be an order of magnitude higher in the case of an appropriately designed torsional cutting head relative to a corresponding longitudinal mode system.
Qualitative transmission characteristics can be simply demonstrated by immersing the head in water. If a longitudinal mode system is immersed in water but with the distal end just above the surface and in air, minimal water disturbance is noted. By contrast, a grooved torsional mode system, providing two exposed normal mode surfaces, shows focused transmission perpendicularly out of the groove. In the absence of such surfaces, so that only shear mode frictional coupling equivalent to the longitudinal system is present, virtually no transmission is observed.
In contrast, transmission from the immersed distal end face of a longitudinal system demonstrates the anticipated high intensity cavitation field with considerable collateral damage potential.
The importance of normal mode interaction between the energised system and the tissue is described by Balamuth U.S. Pat. Nos. 3,636,943 and 3,862,630, in which is described a longitudinal mode vibration being used in conjunction with a clamp, acting parallel to the direction of energy propagation, to grasp tissue and press it onto the vibrating work piece, enabling coupling between the two. The apparatus described in the patents did this at the end face of the work piece.
Ultracision U.S. Pat. No. 5,322,055, follows the clamping principle but the system described actuates the clamp perpendicular to the direction of energy propagation. As described above shear mode interaction only is utilised and this requires high clamping pressure. The present invention applies clamping parallel to the direction of energy propagation but does so via a groove along the distal side of the work piece.
It is one object of the present invention to provide an oscillatory system so that the activated cutting head vibrates in a torsional mode with minimal compression wave transmission beyond the distal extremity.
Although less easily achieved within the necessary constraints associated with surgical applications, there are clear advantages to be gained from the use of torsional mode radiation. Use of torsional mode vibrations is more efficient since maximum coupling can be achieved by transmission into tissue in a direction normal to the axis of the instrument. In the case of longitudinal mode vibrations, normal mode transmission occurs along the length of the instrument only by virtue of frictional effects (see above).
Use of torsional mode vibrations is also safer since energy is absorbed in the target tissue and not transmitted along a probe axis into distant regions.
Also, a torsional mode transducer is more easily accommodated within a scissors handgrip (such as is preferred by surgeons). It therefore gives ergonomic design advantages, and it is a further object of the invention to provide a cutting device having scissors type action.
Higher cutting efficiency will require less electrical power input allowing smaller energy converters and no need for cooling systems.
According to the present invention, there is provided a surgical tool comprising means to generate ultrasonic torsional mode vibrations, a waveguide operatively connected at a proximal end to said generating means and extending a distance therefrom of nxcexT/2 (where xcexT is the wavelength of ultrasonic vibration in the material of the waveguide) to a distal end provided with cutting and/or coagulating means.
The cutting means may comprise a torsionally vibratable element connected to said waveguide in combination with a static element.
Preferably, the means to generate ultrasonic torsional mode vibrations comprises a conversion horn and at least one axial mode driver mounted substantially tangentially thereto.
Advantageously, a shroud is provided to surround and isolate the waveguide along at least a portion of its length.
In this case said static element of the cutting means may be mounted to said shroud, whereby it is isolated from said torsional vibrations.
Preferably, said cutting means has a cutting face between said static and vibrational elements which is normal to the general direction of said torsional vibrations.
A pair of cutting faces normal to the direction of said torsional vibrations may be provided, said pair of faces intersecting at or adjacent an axis of said waveguide.
The cutting means may comprise a plurality of cutting faces, at least one of which is substantially normal to the general direction of said torsional vibrations.
Alternatively or additionally the cutting means may comprise a plurality of cutting faces, at least one of which is substantially parallel to the general direction of said torsional vibrations.
Alternatively or additionally, at least one cutting face may be angled with respect to the general direction of said torsional vibrations, whereby it is acted upon by both normal and parallel components thereof.
In either of the first or second aspects said cutting means may be aspirated by means of a passage extending along or parallel to the waveguide to a vacuum source.